Stormwater runoff from parking lots and streets is often run to catch basins which are interconnected by underground pipes which discharge the water somewhere. In the past, the discharge point was often a low surface elevation point of the surrounding terrain or a nearby water course. However, in recent years environmental regulations have required that the water only be gradually flowed into a stream or natural water course. Thus, in one approach, to which the present invention relates, water is flowed to a specially constructed underground reservoir, in particular to an array of pipes or chambers which has been buried within porous medium such as crushed stone. A familiar subterranean storm water system is comprised of rows of arch shape cross section molded chambers. See Moore et al. U.S. Pat. No. 5,890,838 and Maestro U.S. Pat. No. 6,361,248 and literature from Stormtech LLC, Wethersfield, Conn., U.S., for examples of storm water systems. Stormtech® chambers of the present assignee are a well known commercial product used to construct such systems.
The so-called “first flush,” or initial quantity of surface water which runs off into the drainage system, often carries with it suspended solids, such as sand, sticks, paper, and other debris which has accumulated on the surface since the last storm. Larger suspended or entrained matter is typically removed from the water by surface gratings and catch basins. Sometimes the water is also flowed through a commercial separator device, such as hydrodynamic separator, to further remove suspended material. In most instances, solids, particularly dirt and fine sand, will inevitably be carried along further and will accumulate with time in subterranean storm water structures. Ultimately the systems can become be clogged with the debris, and thus they have to be periodically cleaned.
An improvement in underground stormwater system for dealing with entrained matter, and with which the present invention is useful, comprises an Isolator™ row of chambers. An Isolator row is a solids-retention subsystem of the underground storm water receiving system, like that described in U.S. Pat. No. 6,991,734 of J. Smith et al. A row of chambers which is spaced apart from the main array of underground chambers receives first the water from the drainage system. The Isolator row captures the preponderance of entrained or suspended solids in the water, and the water then flows through the surrounding crushed stone medium into the other chambers of the system. The Isolator row system makes it easier to maintain a stormwater system, since solids in unknown quantities are not distributed about the often-large and difficult-to-access array of chambers.
In order to maintain the function of a underground storm water systems, with or without the use of an Isolator row, periodic cleaning is necessary. If the accumulated solid matter is not removed, then the useful volume of the system and the retention/detention capacity can be unacceptably reduced. To maintain the advantage an Isolator row provides, or to maintain the function of the whole system when it is not present, it is necessary to periodically remove debris. Commonly, debris is removed by devices suited to jet loose and suction away the material. While cleaning can be done on a periodic scheduled basis based on experience, it is more efficient and economic to inspect the system periodically to see if and when cleaning is needed. Inspection in the past has been carried out by use of access ports. A maintenance worker can look down, or probe through, an access port which extends vertically to the surface above the system. The worker can also enter the system by means of manholes at the ends of each row when they are provided However, it is a problem that the inspection itself may not be timely. The rate at which debris accumulates can vary widely over time or from one installation to the next. And accessing the system can interfere with the ordinary use of the overlying surface, can take too much time, and can be shunned by workers, particularly in bad weather. The lack of a good monitoring of sediment within a system can risk failure of the system to function effectively, and could produce untoward effects that are costly to remedy.
While it might seem logical to install an automatic monitoring system, there are a number of factors which make unusual demands on the monitoring system. Typically, they include that the system is outside, that it may not be conveniently located near a power source, that there can be turbulent swirling flow of water and debris within the chamber, that the water may contain salt used for de-icing and therefore be corrosive, that small creatures may visit the system and gnaw on instruments, and that the powerful devices used for emptying sediment could cause damage to any instrumentation.
Thus, it is an object of the present invention to provide a means for inspection or monitoring of the sediment within a storm water system, which means is accurate, simple, reliable, and economic to build and maintain. A further object is to provide a means for monitoring which provides a continuing indication of the system's sediment condition to an unskilled person. A further object is to provide a means which requires minimal or no maintenance, and withstands other maintenance procedures.